tuning a 500 inch stroker for 91 pump gas

[68charger440]

I have a 440 bored 40 over with a balanced 500 inch stroker kit installed
Stock fresh 906 heads. 
10.52 static compression.
edelbrock performer rpm intake
Edelbrock performer cam
Holley 750 carb
Stock converter
3.91 sure grip
18 initial timing
5280 foot altitude here in Denver
My problem is that I can not easily get anything better than 91 octane anymore by my house.  When I used to run 93 I was OK, but now I can not get rid of the ping when I get on it running 91.
Suggestions?

[firefighter3931]

Try backing the total timing off 3-4* and see how that works. 

Ron

[Cooter]

I'm at 10.8:1 and iron heads. Car won't hardly start with 93 when hot.
Good luck, but I see aluminum heads in your future, or one heluva solid cam with loads of duration.

[Kern Dog]

Even the biggest Edelbrock cam is a mild one. My guess is that your cranking compression is at or above 190. Iron heads with that compression isn't going to play well with a mild cam.
I have tried the advice of ...swap in a bigger cam with a later intake closing and it wasn't enough for me. I ended up switching to thicker head gaskets to get the compression down to around 10 to 1.

[fy469rtse]

yep i agree with cooter, i see aluminum heads in the future,
thats the next thing Ron is going to mention , 

[firefighter3931]

Guys, he's at 5000+ ft of elevation.....that makes a big difference   It ran good with 93 but detonates with 91 so he's not that far off   With the current configuration there are two options :

(1) back the timing off a few degrees
(2) spike the pump gas with some race fuel

Of course a set of closed chamber aluminum heads would make a big difference as well.....if that is an option.


Ron

[6spd68]

Is mixing lead an option?

[68charger440]

Thanks to everyone for the input.  I have played with the timing but no go.  Even when running the 93 octane, one tank would be fine and the next would have a slight ping when I got on it, so 93 had me right on the edge.  Aluminum heads might be possible down the rode but not right now. 
If I did go down that road wouldn't most closed chamber heads have a smaller cc and thus increase my CR more than the heat dissipation would help?  I don't know of any aluminum heads that have anything close to the 88 cc volume the 906 has.  Do any of you know of any opened or closed chambered heads that have anything close to the 88 cc volume?  Or is that not as important as I was led to believe it was?
As far as the head gasket goes... How thick of a head gasket can I use and/or maybe some sort of a spacer?
I am not opposed to fuel additives if cheap, but this car is 99% grocery getter so I don't want to bump my fuel costs to $5 a gallon just for running around town.
I am also not opposed to a cam change if the overlap would lower the CR enough, but I don't want anything to radical since I do drive it mostly around town. 

[Kern Dog]

The bigger cam doesn't reduce the static compression ratio but it does change the dynamiccompression ratio.
The standard & common Fel Pro head gasket is .039 compressed. I went with a .075 Cometic. It lowered my squeeze almost a full point and stopped the knocking. I can run 91 octane and hammer the car without knocking.

[68charger440]

The bigger cam doesn't reduce the static compression ratio but it does change the dynamiccompression ratio.
The standard & common Fel Pro head gasket is .039 compressed. I went with a .075 Cometic. It lowered my squeeze almost a full point and stopped the knocking. I can run 91 octane and hammer the car without knocking.

Wow, I didn't think I could get nearly a whole point with just the head gasket change. That sounds both inexpensive as well as effective.  Thanks!!! 
Just to help me understand, why wouldn't the cam change the static CR since the piston would be further up the cylinder before the valves are closed and would have less volume at the beginning of the compression stroke?

[justcruisin]

The static compression ratio doesn't change with cam timing, it is based on the swept volume of your cylinder in relation to the sum volume of the head chamber, piston compression height, piston valve relieves and head gasket without consideration to cam timing. The dynamic ratio factors in cam timing.

[firefighter3931]

I don't know of any aluminum heads that have anything close to the 88 cc volume the 906 has.  Do any of you know of any opened or closed chambered heads that have anything close to the 88 cc volume?  

Edelbrock makes an 88cc open chamber RPM head. 

http://www.summitracing.com/int/parts/edl-60189/overview/

When the time comes to install the new heads you might want to consider upgrading the cam....the one you're using is small for a 500in build and is increasing the dynamic compression too much. 

Have you done a compression test ?

Do you have the cam specs handy ?


Ron

[68charger440]

I don't know of any aluminum heads that have anything close to the 88 cc volume the 906 has.  Do any of you know of any opened or closed chambered heads that have anything close to the 88 cc volume?  

Edelbrock makes an 88cc open chamber RPM head. 

http://www.summitracing.com/int/parts/edl-60189/overview/

When the time comes to install the new heads you might want to consider upgrading the cam....the one you're using is small for a 500in build and is increasing the dynamic compression too much. 

Have you done a compression test ?

Do you have the cam specs handy ?


Ron

Thanks everyone again
I haven't done a compression test in a couple of years and can't find the sheet I wrote them down on when I did.  I'll try to find some time to do that in the next few weeks.  I knew the performer cam was not good but I had it in the garage from a different motor I built years ago, and figured until I got around to the heads it would be better than stock.  I attached the cam specs below.

[BSB67]

You should first do a cranking cylinder pressure test.  This will help us where you are at relative to the altitude. 

Things that can help: Cooler plugs, cooler thermostat, make sure you don't have a lean condition, heat crossover blocked, slower timing curve.

Your cam is pretty small too.  Larger cam with a larger LSA will help your cranking cylinder pressure but maintain a very docile idle.

[Cooter]

You musta been after a 500 inch motor with a stock idle with a cam that short.

[68charger440]

You musta been after a 500 inch motor with a stock idle with a cam that short.

I will get that compression test as soon as I can, but I am recovering from neck surgery from c4 thru c7 that went wrong and left me pretty weak and with some partial paralysis so what was once a piece of cake is now a struggle.  Even something as simple as a compression test can wreck me for a couple of days.  Enough whining, I just wanted to let you know why it might take me a while to get it done.  Maybe I'll shame my son into coming over to do it.  As for the cam mismatch, like I said earlier it was in my garage when I built the motor and I threw it in there while I decided what I wanted to do with the top end.  Then my health went downhill and everything stalled out.

[firefighter3931]

Sorry to hear about your health issues....you have a lot on your plate to deal with. Best wishes for a full recovery. 

Based on the cam specs combined with the static compression it's clear why you're having these issues. That cam would be considered very mild for a 440 let alone a 500ci build. Realisticly, you should have at least another 30-40* of duration @ .050 valve lift with that combination.

The cranking compression numbers will tell the whole story.


Ron

[69white hat]

If this is a stupid question please forgive my ignorance.

Wouldn't a longer duration cam, which allows the piston to move up more before the valves shut and compression begins, just force a bunch of o2 and fuel out of the cylinder causing less power and poor mileage?

It makes sense in my brain, but I don't get complimented on my brain a lot, LOL. Please, someone explain how it really works, as I have the same problem.
I have 160 cranking compression, but not a clue whats in the 440. I have edelbrock aluminum rpm heads. The timing is turned way down to reduce pinging and when I decide to rebuild the motor I want to know what duration has to do with it.

Thanks.

[Kern Dog]

That is a REALLY mild cam!

In short:
Static compression is easily measured with a guage. ALWAYS do this with all 8 spark plugs removed. Anything over 190 will be a struggle to avoid detonation. You can try colder spark plugs, back off the timing, remove the vacuum advance and/or only run the car in cool weather but you will always be dancing on the edge of the cliff.
Dynamic compression is a calculated number based on a few factors. The theory is that you can have 8, 10 or even 50 to one compression but that actual compression doesn't begin until the intake valve closes. As the piston passes Bottom dead center and rises, the LATER the intake valve closes, the LOWER the dynamic compression. There are several online calculators to determine the dynamic ratio. Again, the belief is that you need to be at 8.0 or less to run knock free on 91.
Without going into too much depth regarding camshaft science, the later intake closing does make the engine idle rougher BUT it is a tradeoff. IF the heads have the flow capability, the loss in low rpm idle quality is usually MORE than offset by higher rpm power.
White Hat:
With 160 cranking compression, the only reason I could imagine that you are knocking is that the spark timing is too far advanced or that you are using regular or mid grade fuel. Not sure of your ignition, but The Mopar Performance distributors included in their electronic conversion kits had a fast and long advance curve. If a man set his initial to 15 degrees, the 28 degrees of mechanical advance (Standard procedure with MP dist.) puts you at 43 degrees! I don't know of any Wedge that needs that much timing. I had this problem. If I set the initial to where the idle quality was good, the car knocked badly on full throttle. If I retarded the timing to stop the knock, it idled as if I pulled 5 plug wires off. I had to dig into the distributor to limit the amount of advance that the distributor would deliver. In my case that was 16 degrees.

[68charger440]

Ok, I found my cranking compression numbers from a couple of years ago.  Its roughly 160 across the board. 

On a side bar question...  I have my original Holley 3918 750 carb that I had gone through awhile back and never put it on the car.  I was thinking of putting it on since I have never been happy with my idle with the carter 750 afb that is on there now.  The original Holley has got 65 jets and a power valve is marked going clockwise E 8 5 2 that came with the rebuild kit. Is that an 8.5? The old PV is marked 7.4 7.  If I remember right my vacuum was in the mid teens when idling in drive.  Before I put it on the car do you have any suggestions on the jets and PV to start with or should I just start with these stock jets and PV for now?

[68charger440]

Guys... hold of on using the 160 cranking compression I posted earlier.  That sheet I found might have been from a different motor.

[68charger440]

OK I pulled ONE plug, not all of them and tested that cylinder with WOT.  It read 163 so I think the sheet I found was the correct one.  Adjusted for altitude it would get me to 185-190 range, so I think the 160 to 165 range is correct for me being over a mile high in altitude.

[firefighter3931]

The 160ish cranking pressure is in the ball park. It certainly could be a fuel delivery issue (too lean) under load. That carb you listed above is a 4160 style vac secondary holley which is basicly designed for a stock 440. You might be able to jet it up enough to make it work ; 72/80 with a 6.5PV would be a good starting point.

A wideband O2 would really help in this case. If it's going way lean on the AFR under load the readings will confirm this.



Ron

[firefighter3931]

OK I pulled ONE plug, not all of them and tested that cylinder with WOT.  It read 163 so I think the sheet I found was the correct one.  Adjusted for altitude it would get me to 185-190 range, so I think the 160 to 165 range is correct for me being over a mile high in altitude.

Your calculations appear to be spot on.    At 5000ft the correction factor is .8617

190 psi x .8617 = 163.72 psi

I'd start looking at the carb after you verify that your total timing is at 36*

What head gasket are you using ?


Ron

[68charger440]

OK I pulled ONE plug, not all of them and tested that cylinder with WOT.  It read 163 so I think the sheet I found was the correct one.  Adjusted for altitude it would get me to 185-190 range, so I think the 160 to 165 range is correct for me being over a mile high in altitude.

Your calculations appear to be spot on.    At 5000ft the correction factor is .8617

190 psi x .8617 = 163.72 psi

I'd start looking at the carb after you verify that your total timing is at 36*

What head gasket are you using ?


Ron

The head gasket is a Rol .044 compressed
The carb has always been an issue for me since I stink at dialing them in.  I have three choices.
The Carter competition 750 that is on the car right now.  Doesn't idle at low rpm very well.
A Holley Street Avenger 870 That probably needs a rebuild
The stock rebuilt 750 that I mentioned earlier in this thread.
I never could get the street avenger or the carter to idle smooth at low rpm.  That is why in desperation I am asking about the original Holley 750. 
Getting a smooth idle at 700 or 800 rpm should not be a problem with my current configuration on the motor?  Isn't that Right?